I ordered the XL2420T. I'm just worried my GTX 660 might not be able to output 120fps at 120hz.

One note; the XL2420T will have some slight overdrive artifact during LightBoost that doesn't show up on XL2411T or VG248QE. That said, the XL2420T has much better color than the XL2411T and the VG248QE. I'd love to hear back about your experience.